The use of blankets in printing techniques such as, for example, offset lithography, is well known, wherein such blankets have a primary function of transferring ink from a printing plate to paper. Such printing blankets are very carefully designed so that the surface of the blanket is not damaged, either by mechanical contact of the blanket with the press or by chemical reaction with the ink ingredients or other solvents used in the printing process. Repeated mechanical contacts do cause a certain amount of compression of the blanket, however, which must be maintained within acceptable limits so that the image is properly reproduced. It is also important that the blanket have resiliency, i.e., that it be capable of eventually returning to its original thickness, and that it provide image transfer of a constant quality regardless of the amount of use to which the blanket is put.
Printing blankets typically comprise, on their lower surface, a substrate or base material which provides integrity to the blanket. Woven fabrics are preferred for forming this base. The base may consist of one or more layers or plys of fabric (the terms "layer" and "ply" are used interchangeably herein). The printing, or "working" surface at the top of the blanket, i.e., the surface that actually contacts the ink, is usually a layer of an elastomeric material such as rubber. As used herein, the terms "upper" or "top" relate to that portion of an individual ply, or of the blanket itself, furtherest removed from the cylinder of the printing press when the blanket is installed thereon. Alternately, "lower" or "bottom" is used to refer to those portions of either an individual ply or the blanket which would be most closely adjacent the cylinder upon installation of the blanket.
The printing surface is conventionally made by calendering or spreading rubber in layers upon the base ply until a desired thickness of the material has been deposited, after which the assembly is cured, i.e., vulcanized, to provide the finished blanket. Such blankets are acceptable for many applications, but they often lack the necessary compressibility and resiliency needed for other applications. It is desirable, therefore, to produce more highly compressible blankets with improved resiliency.
It is difficult, however, to obtain such improved compressibility using the standard construction described above because the rubber material, while highly elastomeric, is not compressible; that is, it cannot be compressed in a direction at right angles to its surface without causing a distortion or stretch of the blanket in areas adjacent to the point of compression. If irregularities exist in the printing plate, the press, or the paper, the compression to which the blanket is exposed will vary during operation of the press and the irregularities will be magnified by the lack of compression in the printing blanket.
Therefore, a key to obtaining a printing blanket having the desired compressibility and resiliency is in providing a compressible layer therein. In particular, it is generally known that by including at least one layer of material comprising a compressible layer of resilient polymer in a printing blanket, that printing problems such as those described above, as well as "blurring" (i.e., a lack of definition), caused by a small standing wave in the blanket printing surface adjacent to the nip of the printing press, can be avoided. Such compressible layer also can serve to absorb a "smash", that is, a substantial deformation in the blanket caused by a temporary increase in the thickness in the material to be printed due to, for example, the accidental introduction of more than one sheet of paper during the printing operation. By incorporating a compressible layer in the blanket, a "smash" can be absorbed without permanent damage to the blanket or impairment of the printing quality of the blanket. In addition, a resilient, compressible layer helps to maintain the evenness of the printing surface and the thickness of the blanket during the printing operation by restoring the normal thickness of the blanket after compression at the nip of the press.
Many different ways of producing a compressible layer within a printing blanket are known in the art. For example, compressible layers have been formed by mixing granular salt particles with the polymer used to produce the layer, and thereafter leaching the salt from the polymer to create voids therein. Such a method is disclosed in Haren et al. U.S. Pat. No. 4,025,685. The voids in the underlying compressible layer thus permit positive displacement of the surface layer without causing distortion thereof since volume compression occurs and displacement takes place substantially perpendicularly to the impact of the press.
Other methods, such as the use of compressible fiber structures, have also been tried heretofore to produce compressible layers. Examples are found in Duckett et al. U.S. Pat. Nos. 3,887,750 and 4,093,764. Rodriguez, U.S. Pat. No. 4,303,721 teaches a compressible blanket made using blowing agents to create voids in the compressible layer. A further method, involving the use of rubber particles to create voids, is disclosed in Rhodarmer U.S. Pat. No. 3,795,568.
Forming voids with the use of blowing agents has the disadvantages, however, that the size of the voids to be formed, and the interconnection of such voids, is not easily controlled. Oversized voids and interconnected voids cause some areas of the printing blanket to be more compressible and less resilient than adjacent areas, which results in the occurrence of deformations during printing. Moreover, the salt leaching technique described above also has disadvantages in that the particle sizes used are limited, and the process is difficult, time consuming and expensive.
More recently, it has been found preferable to produce printing blankets having a compressible layer comprising a cellular resilient polymer having cells or voids in the compressible layer formed with the use of discrete microspheres. It has been found particularly advantageous to produce a compressible layer by incorporating hollow thermoplastic microspheres in the polymer, as illustrated by Larson U.S. Pat. No. 4,042,743. These microspheres are resilient and thus impart good compressibility properties to the layer.
However, in prior art methods of producing a compressible layer employing thermoplastic microspheres for a printing blanket, it has been found that the thickness of the compressible layer to be formed is not easily controlled since typical thermoplastic microspheres will melt at normal processing and vulcanizing temperatures. Since the microspheres melt before the vulcanization is complete, and before the compressible layer achieves a set structure, agglomeration of the voids created by the microspheres occurs, and size variations in the voids also occur. This can affect the overall performance properties of the blanket. Also, the variations in the sizes of the voids can weaken the printing blanket, causing it to wear out prematurely.
Gaworowski et al. U.S. Pat. No. 4,770,928 attempted to solve these problems by incorporating into the elastomeric compounds utilized to form a matrix for the microspheres within the compressible layer, an accelerator capable of permitting vulcanization of the elastomeric compound at a temperature below the melting point of the microspheres. The use of such relatively low temperatures during the vulcanization process, however, results in the need for additional periods of vulcanization with a concurrent increase in the cost, i.e., including that of the accelerator, and complexity of blanket manufacture.
Shrimpton et al. U.S. Pat. No. 3,700,541 and its corresponding British patent No. 1,327,758 disclose that microspheres made of high temperature thermosetting plastics allow the layer to be cured using conventional high temperature vulcanization processes. However, these microspheres are less resilient than thermoplastic microspheres, so that compressibility properties of the layer are compromised.